Here is an interesting article by James Alt, David Dreyer Lassen and Joachim Wehner that might help explain why it's hard for governments to contend with the problem of tax planning (or base erosion, or however one might like to categorize the issue): governments themselves are inclined toward playing around with numbers, too, or what the authors call engaging in gimmickry. It's also another compelling argument for more transparency in governance coupled with an example of why governments are ambivalent about meeting that goal. Abstract:

This article analyzes the political origins of differences in adherence to the fiscal framework of the European Union (EU). It shows how incentives to use fiscal policy for electoral purposes and limited budget transparency at the national level, combined with the need to respond to fiscal rules at the supranational level, interact to systematically undermine the Economic and Monetary Union through the employment of fiscal gimmicks or creative accounting. It also explains in detail how national accounts were manipulated to produce electoral cycles that were under the radar of the EU budget surveillance system, and concludes with new perspectives on the changes to (and challenges for) euro area fiscal rules.

And here are some observations from the paper:

We show that: (1) despite reporting rules and an elaborate monitoring mechanism (including Eurostat), political incentives resulting from the electoral cycle and the state of the economy systematically undermined compliance with SGP fiscal rules; (2) under such rules, the scale of gimmickry depends on the degree of fiscal transparency in the domestic budget process; (3) incentives for fiscal gimmickry grew with the adoption of these fiscal rules, and tampering with accounting related to subsidies was not the only way in which countries evaded the SGP and Eurostat supervision; and (4) contrary to a good deal of contemporary discussion, non-compliance with the SGP was not 'all about Greece'. Greece was indeed an extreme case, the least transparent of the countries we study. However, the patterns we identify appear whether or not we include Greece in the data.

When larger deficits loomed in an economic downturn, low-transparency countries also systematically circumvented the reporting rules using creative accounting.

Our result – that despite common supranational rules and monitoring, domestic institutions (budget transparency), politics (elections) and economic cycles (recessions) explain much of the variation in outcomes – reinforces the argument that 'the source of fiscal discipline is at the domestic level'.

...asymmetric information in an economic union is not only of academic interest, but has serious, real-world consequences for sustaining co-operation among national governments.

...Why would governments choose to misrepresent the state of their public finances? Euro member countries generally face three audiences: domestic voters, bond markets and the EU itself. Conceptually, countries projecting deficits or debt levels that violate the SGP rules can – for a given level of budget transparency – do three things. Each involves a different trade-off. First, they can observe the fiscal rules and make real adjustments to tax and expenditure levels, which will placate bond markets and Eurostat, but will be costly if the resulting policies are unpopular with voters at the national level. Secondly, they can forego fiscal consolidation, break the rules outright, and post deficits and debts in excess of SGP thresholds. This also can come at a price. Greece's entry into the common currency was delayed due to too-high deficits and, after the introduction of the euro, the system had penalties that made it potentially costly for countries to violate the rules. Thirdly, countries can resort to gimmickry, leaving real outcomes (especially spending) unchanged.8 Voters are unharmed in the short run, and gimmicks fool bond markets and supranational authorities to the extent they are undetected. Here the trade-off is intertemporal: if undetected, gimmickry keeps governments on good terms with everyone in the present, but may entail considerable costs, if deficits and debts later accumulate, in the form of high bond yields and even political unrest. Strategic choice could involve more than one of these avenues for action.

...If countries face costly constraints – either politically, from voters, or economically, from supranational fiscal rules or markets – why would countries not simply reduce transparency in order to facilitate fiscal gimmickry? They could, but making governance structures less transparent is visible, carries substantial reputational costs and, in our context, is penalized by bond markets.

...how can we make gimmickry observable and quantified, when the point of misrepresenting fiscal quantities is to avoid detection? We next show how gimmickry can be inferred from traces left in the national accounts, even after Eurostat scrutiny.

...One measure of gimmickry that is reasonably well known to practitioners is the stock-flow adjustment (SFA). The SFA is a statistical residual, an accounting item defined to reconcile the difference between a change in a government's debt (the total face value of the 'debt-like' or 'fixed' claims held against it) and budget deficit (the excess of spending over revenue)

...shares and other equity' transactions become gimmicks when, for instance, payments to cover recurring losses by a state-owned company are treated as equity purchases instead of current transfers.

...A government's electoral incentives are captured by years left in its term of office, ending in zero in the election year: there should be more gimmickry when fewer years are left

...Panel A clearly reveals an electoral cycle in gimmicks which is conditional on transparency.But increasing transparency reduces or eliminates the electoral cycle.

... Appendix 8 presents estimates with different measurements and coding of rules, transparency and other variables, varied samples and including a lagged dependent variable. Those tests qualitatively support our main results: the presence of an electoral cycle in gimmicks, more pressure from hard times and recourse to gimmicks exacerbated by rules – all of which are conditional on limited transparency.

Moreover, supranational fiscal rules that are intended to sustain co-operation instead exacerbate incentives for national governments to manipulate reported data rather than fix fiscal policy.

These are systematic tendencies, rather than the actions of any single country. Budget process transparency can reduce the incentives to manipulate, even those that would otherwise intensify in times of economic stress. Warnings that have been raised in policy and research papers since the early 1990s about the risks of moral hazard in economic policy making for countries in economic unions remain a concern.

The following observation meshes with other work on the tendency of governments (such as here and here and here) to deficit-finance more generally:

In democracies, even advanced ones, politicians' incentives to employ gimmicks get stronger as elections approach.

And finally the punchline:

...The results show that Greece was not a special case; rather, it was the extreme case of a general, and comprehensible, pattern.